1. Technical Field
The disclosed subject matter relates to a light emitting diode (LED) light source for a vehicle lighting device, for example, to be used as a headlight, an auxiliary light, spotlight, traffic light, etc. for a vehicle or other application, and a vehicle lighting device using the above LED light source.
2. Related Art
Conventionally, such an LED light source is configured, for example, as shown in FIG. 9. Namely, in FIG. 9, an LED light source 1 includes at least one LED chip 3 (in the case shown in the figure, four LED chips) placed on a base 2, a reflector 4 disposed on the base 2 so as to surround the LED chip(s) 3, and a phosphor layer 5 filled in a hollow 4a of the reflector 4.
The above base 2 is formed of an insulating material having satisfactory thermal conductivity, such as copper, ceramic (AlN, alumina Al2O3) and silicon (Si).
Each LED chip 3 is mounted on a chip mounting portion having a conductive pattern formed on the base 2 by means of die bonding, etc., and electrically connected to an adjacent connection portion of a similar conductive pattern with a bonding wire 3a. 
The reflector 4 is configured such that at least the internal face of the hollow 4a thereof has a light-shielding property. The hollow 4a of the reflector 4 vertically penetrates so as to surround the entire LED chips 3 on the periphery thereof.
Here, the reflector 4 is configured such that the internal face of the hollow 4a thereof constitutes a reflecting face.
The phosphor layer 5 is formed of a light-transparent material of, for example, silicone, and has a particulate phosphor (not shown) mixed therein, so as to be dispersed substantially uniformly.
Here, the phosphor is excited by light emitted from each of the above-mentioned LED chips 3, and generates fluorescence having a different wavelength as compared to that emitted by the LED chips 3.
Here, the above LED chip 3 and the phosphor are used with the following combinations.
Namely, for example, to obtain white light, a blue LED chip can be used for outputting blue light, and a phosphor can be used for generating yellow light by means of excitation by the blue light from the blue LED chip. Color mixing of the blue color light from the LED chip with the yellow color light from the phosphor occurs and white light can be obtained in a simulated manner.
Similarly, to obtain white light, an ultraviolet LED chip can be used for outputting ultraviolet light, and an RGB phosphor can be used for generating substantially visible white light triggered by the ultraviolet light emitted from the ultraviolet LED chip.
According to the LED light source 1 having the aforementioned structure, the light output from the LED chip 3 is output toward the upper direction via the phosphor layer 5, either directly or after being reflected on the internal wall of the reflector 4.
At this time, for example, when blue light output from the LED chip 3 hits the phosphor in the phosphor layer 5, the phosphor absorbs the blue light, and generates yellow light as fluorescence through wavelength conversion. Then, color mixing of the yellow light with the blue light from the LED chip 3 occurs and white light is output in a simulated manner.
As the output becomes high in the LED light source 1 having the above-mentioned structure, a large current of the order of a few hundred MA flows in the LED chip 3. Thus, a large amount of heat generation occurs.
Among the power input to the LED chip, approximately 85% is converted to heat, and the efficiencies of both the LED chip 3 and the phosphor tend to degrade as the temperature rises. Therefore, it is helpful to efficiently radiate the generated heat to the outside.
In particular, the phosphor has a marked efficiency degradation caused by the temperature rise. For example, when the temperature rises by 50° C. from 50° C. to 100° C., in general, the fluorescence conversion efficiency thereof is decreased by 10% or more.
Further, as described above, because the phosphor surrounds the LED chip 3 in a state including silicone, etc. having a relatively low coefficient of thermal conductivity in the phosphor layer 5, the heat is apt to be confined internally. Also, because the phosphor generates heat by absorbing the light, the phosphor tends to have a higher temperature than the LED chip 3.
Therefore, in the LED light source 1 that outputs white light, for example, as shown in FIG. 10, as the temperature rises, the blue light (peak A) from the LED chip 3 is reduced, and also the yellow light (peak B) from the phosphor is reduced to a larger extent.
Accordingly, as shown in FIG. 11, with the increase of temperature, the chromaticity greatly deviates, for example, from the area of “ECE No. 99” shown by the symbol D, although staying, for example, in “an SAE white area” shown by the symbol C.
In contrast, increased output LED light sources have been considered for utilization in a variety of fields, and in some cases have already been put into practical use. A vehicle headlight for an automobile is one such use.
In the case of the automobile headlight, a lot of heat generation occurs as a result of the engine heat. Accordingly, the temperature in the ambient circumstance for the vehicle headlight may be extremely high. For example, at an idling time, the ambient temperature may reach 70° C., or even higher.
When using a white-light LED light source that produces a high output in such an ambient circumstance, the temperature of the LED chip 3 may exceed 100° C. Accordingly, the LED light source 1 can itself be configured as a low heat-resistance package, and can be combined together with a structure enabling larger heat radiation, such as a heat sink.
For example, when the heat resistance of the package of the LED light source 1 is 3° C./W, the heat resistance of the heat sink is 5° C./W, and the power consumption of a single LED light source 1 is 5 W, then, the temperature of the LED chip 3 rises by 35° C. to the external air temperature. Therefore, when the ambient temperature of the vehicle headlight is 70° C., the temperature of the LED chip 3 becomes 105° C., and the phosphor temperature can exceed 120° C.
Thus, when efficiency is degraded due to the temperature rise, the energy not converted to light is converted to heat, which causes a further increase in the heat generation amount, resulting in a further rise of temperature and further degradation.
To cope with the above problem, Japanese Patent Application Laid-open No. 2000-261039 (the JP'039 publication) discloses a light source device in which the heat generated by an LED chip is radiated to the outside via an electrode extending to the external surface of a substrate.
Now, according to the light source device disclosed in the JP'039 publication, it is not possible to perform sufficient radiation when the ambient temperature is high, and accordingly, it is not possible to efficiently radiate the heat generated by the LED chip.
To solve the above problem(s), it is effective to set an optimal drive current value for the LED chip, based on the temperature of the LED chip or the phosphor. Conventionally, the drive control of the LED chip has been performed by measuring the ambient temperature of an LED light source package using a temperature sensor, etc.
However, the ambient temperature of the LED light source package has a time lag as compared to the temperatures of the LED chip and the phosphor. Since the temperatures of the LED chip and the phosphor have not been measured accurately in real time, it has been difficult to drive the LED chip with an optimal drive current value. As a result, the temperatures of the LED chip and the phosphor further rise, which results in a vicious circle of further efficiency degradation.
Moreover, conventionally, based on a reference heat resistance value of a standard package, the temperature of the LED chip or the phosphor, a heat source, is estimated by detecting an ambient package temperature, a heat sink temperature, or the like. However, there may be cases in which the estimated temperature greatly differs from the actual temperature because the heat resistance of the package greatly differs among the packages of the same structure, depending on the junction conditions of the LED chip and the package. Therefore, it has been difficult to detect the temperatures of the LED chip and the phosphor with accuracy.
Also, it has been required to manage the phosphor temperature for efficient utilization of the LED using the phosphor.